Content-type: text/html Man page of List::MoreUtils

List::MoreUtils

Section: User Contributed Perl Documentation (3pm)
Updated: 2009-07-26
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NAME

List::MoreUtils - Provide the stuff missing in List::Util  

SYNOPSIS

    use List::MoreUtils qw(any all none notall true false firstidx first_index 
                           lastidx last_index insert_after insert_after_string 
                           apply after after_incl before before_incl indexes 
                           firstval first_value lastval last_value each_array
                           each_arrayref pairwise natatime mesh zip uniq minmax);

 

DESCRIPTION

"List::MoreUtils" provides some trivial but commonly needed functionality on lists which is not going to go into "List::Util".

All of the below functions are implementable in only a couple of lines of Perl code. Using the functions from this module however should give slightly better performance as everything is implemented in C. The pure-Perl implementation of these functions only serves as a fallback in case the C portions of this module couldn't be compiled on this machine.

any BLOCK LIST
Returns a true value if any item in LIST meets the criterion given through BLOCK. Sets $_ for each item in LIST in turn:

    print "At least one value undefined"
        if any { !defined($_) } @list;

Returns false otherwise, or "undef" if LIST is empty.

all BLOCK LIST
Returns a true value if all items in LIST meet the criterion given through BLOCK. Sets $_ for each item in LIST in turn:

    print "All items defined"
        if all { defined($_) } @list;

Returns false otherwise, or "undef" if LIST is empty.

none BLOCK LIST
Logically the negation of "any". Returns a true value if no item in LIST meets the criterion given through BLOCK. Sets $_ for each item in LIST in turn:

    print "No value defined"
        if none { defined($_) } @list;

Returns false otherwise, or "undef" if LIST is empty.

notall BLOCK LIST
Logically the negation of "all". Returns a true value if not all items in LIST meet the criterion given through BLOCK. Sets $_ for each item in LIST in turn:

    print "Not all values defined"
        if notall { defined($_) } @list;

Returns false otherwise, or "undef" if LIST is empty.

true BLOCK LIST
Counts the number of elements in LIST for which the criterion in BLOCK is true. Sets $_ for each item in LIST in turn:

    printf "%i item(s) are defined", true { defined($_) } @list;

false BLOCK LIST
Counts the number of elements in LIST for which the criterion in BLOCK is false. Sets $_ for each item in LIST in turn:

    printf "%i item(s) are not defined", false { defined($_) } @list;

firstidx BLOCK LIST
first_index BLOCK LIST
Returns the index of the first element in LIST for which the criterion in BLOCK is true. Sets $_ for each item in LIST in turn:

    my @list = (1, 4, 3, 2, 4, 6);
    printf "item with index %i in list is 4", firstidx { $_ == 4 } @list;
    __END__
    item with index 1 in list is 4

Returns "-1" if no such item could be found.

"first_index" is an alias for "firstidx".

lastidx BLOCK LIST
last_index BLOCK LIST
Returns the index of the last element in LIST for which the criterion in BLOCK is true. Sets $_ for each item in LIST in turn:

    my @list = (1, 4, 3, 2, 4, 6);
    printf "item with index %i in list is 4", lastidx { $_ == 4 } @list;
    __END__
    item with index 4 in list is 4

Returns "-1" if no such item could be found.

"last_index" is an alias for "lastidx".

insert_after BLOCK VALUE LIST
Inserts VALUE after the first item in LIST for which the criterion in BLOCK is true. Sets $_ for each item in LIST in turn.

    my @list = qw/This is a list/;
    insert_after { $_ eq "a" } "longer" => @list;
    print "@list";
    __END__
    This is a longer list

insert_after_string STRING VALUE LIST
Inserts VALUE after the first item in LIST which is equal to STRING.

    my @list = qw/This is a list/;
    insert_after_string "a", "longer" => @list;
    print "@list";
    __END__
    This is a longer list

apply BLOCK LIST
Applies BLOCK to each item in LIST and returns a list of the values after BLOCK has been applied. In scalar context, the last element is returned. This function is similar to "map" but will not modify the elements of the input list:

    my @list = (1 .. 4);
    my @mult = apply { $_ *= 2 } @list;
    print "\@list = @list\n";
    print "\@mult = @mult\n";
    __END__
    @list = 1 2 3 4
    @mult = 2 4 6 8

Think of it as syntactic sugar for

    for (my @mult = @list) { $_ *= 2 }

after BLOCK LIST
Returns a list of the values of LIST after (and not including) the point where BLOCK returns a true value. Sets $_ for each element in LIST in turn.

    @x = after { $_ % 5 == 0 } (1..9);    # returns 6, 7, 8, 9

after_incl BLOCK LIST
Same as "after" but also inclues the element for which BLOCK is true.
before BLOCK LIST
Returns a list of values of LIST upto (and not including) the point where BLOCK returns a true value. Sets $_ for each element in LIST in turn.
before_incl BLOCK LIST
Same as "before" but also includes the element for which BLOCK is true.
indexes BLOCK LIST
Evaluates BLOCK for each element in LIST (assigned to $_) and returns a list of the indices of those elements for which BLOCK returned a true value. This is just like "grep" only that it returns indices instead of values:

    @x = indexes { $_ % 2 == 0 } (1..10);   # returns 1, 3, 5, 7, 9

firstval BLOCK LIST
first_value BLOCK LIST
Returns the first element in LIST for which BLOCK evaluates to true. Each element of LIST is set to $_ in turn. Returns "undef" if no such element has been found.

"first_val" is an alias for "firstval".

lastval BLOCK LIST
last_value BLOCK LIST
Returns the last value in LIST for which BLOCK evaluates to true. Each element of LIST is set to $_ in turn. Returns "undef" if no such element has been found.

"last_val" is an alias for "lastval".

pairwise BLOCK ARRAY1 ARRAY2
Evaluates BLOCK for each pair of elements in ARRAY1 and ARRAY2 and returns a new list consisting of BLOCK's return values. The two elements are set to $a and $b. Note that those two are aliases to the original value so changing them will modify the input arrays.

    @a = (1 .. 5);
    @b = (11 .. 15);
    @x = pairwise { $a + $b } @a, @b;   # returns 12, 14, 16, 18, 20

    # mesh with pairwise
    @a = qw/a b c/;
    @b = qw/1 2 3/;
    @x = pairwise { ($a, $b) } @a, @b;  # returns a, 1, b, 2, c, 3

each_array ARRAY1 ARRAY2 ...
Creates an array iterator to return the elements of the list of arrays ARRAY1, ARRAY2 throughout ARRAYn in turn. That is, the first time it is called, it returns the first element of each array. The next time, it returns the second elements. And so on, until all elements are exhausted.

This is useful for looping over more than one array at once:

    my $ea = each_array(@a, @b, @c);
    while ( my ($a, $b, $c) = $ea->() )   { .... }

The iterator returns the empty list when it reached the end of all arrays.

If the iterator is passed an argument of '"index"', then it retuns the index of the last fetched set of values, as a scalar.

each_arrayref LIST
Like each_array, but the arguments are references to arrays, not the plain arrays.
natatime BLOCK LIST
Creates an array iterator, for looping over an array in chunks of $n items at a time. (n at a time, get it?). An example is probably a better explanation than I could give in words.

Example:

    my @x = ('a' .. 'g');
    my $it = natatime 3, @x;
    while (my @vals = $it->())
    {
        print "@vals\n";
    }

This prints

    a b c
    d e f
    g

mesh ARRAY1 ARRAY2 [ ARRAY3 ... ]
zip ARRAY1 ARRAY2 [ ARRAY3 ... ]
Returns a list consisting of the first elements of each array, then the second, then the third, etc, until all arrays are exhausted.

Examples:

    @x = qw/a b c d/;
    @y = qw/1 2 3 4/;
    @z = mesh @x, @y;       # returns a, 1, b, 2, c, 3, d, 4

    @a = ('x');
    @b = ('1', '2');
    @c = qw/zip zap zot/;
    @d = mesh @a, @b, @c;   # x, 1, zip, undef, 2, zap, undef, undef, zot

"zip" is an alias for "mesh".

uniq LIST
Returns a new list by stripping duplicate values in LIST. The order of elements in the returned list is the same as in LIST. In scalar context, returns the number of unique elements in LIST.

    my @x = uniq 1, 1, 2, 2, 3, 5, 3, 4; # returns 1 2 3 5 4
    my $x = uniq 1, 1, 2, 2, 3, 5, 3, 4; # returns 5

minmax LIST
Calculates the minimum and maximum of LIST and returns a two element list with the first element being the minimum and the second the maximum. Returns the empty list if LIST was empty.

The minmax algorithm differs from a naive iteration over the list where each element is compared to two values being the so far calculated min and max value in that it only requires 3n/2 - 2 comparisons. Thus it is the most efficient possible algorithm.

However, the Perl implementation of it has some overhead simply due to the fact that there are more lines of Perl code involved. Therefore, LIST needs to be fairly big in order for minmax to win over a naive implementation. This limitation does not apply to the XS version.

part BLOCK LIST
Partitions LIST based on the return value of BLOCK which denotes into which partition the current value is put.

Returns a list of the partitions thusly created. Each partition created is a reference to an array.

    my $i = 0;
    my @part = part { $i++ % 2 } 1 .. 8;   # returns [1, 3, 5, 7], [2, 4, 6, 8]

You can have a sparse list of partitions as well where non-set partitions will be undef:

    my @part = part { 2 } 1 .. 10;          # returns undef, undef, [ 1 .. 10 ]

Be careful with negative values, though:

    my @part = part { -1 } 1 .. 10;
    __END__
    Modification of non-creatable array value attempted, subscript -1 ...

Negative values are only ok when they refer to a partition previously created:

    my @idx = (0, 1, -1);
    my $i = 0;
    my @part = part { $idx[$++ % 3] } 1 .. 8;   # [1, 4, 7], [2, 3, 5, 6, 8]

bsearch BLOCK LIST
Performs a binary search on LIST which must be a sorted list of values. BLOCK must return a negative value if the current element (stored in $_) is smaller, a positive value if it is bigger and zero if it matches.

Returns a boolean value in scalar context. In list context, it returns the element if it was found, otherwise the empty list.

 

EXPORTS

Nothing by default. To import all of this module's symbols, do the conventional

    use List::MoreUtils qw/:all/;

It may make more sense though to only import the stuff your program actually needs:

    use List::MoreUtils qw/any firstidx/;

 

ENVIRONMENT

When "LIST_MOREUTILS_PP" is set, the module will always use the pure-Perl implementation and not the XS one. This environment variable is really just there for the test-suite to force testing the Perl implementation, and possibly for reporting of bugs. I don't see any reason to use it in a production environment.  

VERSION

This is version 0.22.  

BUGS

There is a problem with a bug in 5.6.x perls. It is a syntax error to write things like:

    my @x = apply { s/foo/bar/ } qw/foo bar baz/;

It has to be written as either

    my @x = apply { s/foo/bar/ } 'foo', 'bar', 'baz';

or

    my @x = apply { s/foo/bar/ } my @dummy = qw/foo bar baz/;

Perl5.5.x and perl5.8.x don't suffer from this limitation.

If you have a functionality that you could imagine being in this module, please drop me a line. This module's policy will be less strict than "List::Util"'s when it comes to additions as it isn't a core module.

When you report bugs, it would be nice if you could additionally give me the output of your program with the environment variable "LIST_MOREUTILS_PP" set to a true value. That way I know where to look for the problem (in XS, pure-Perl or possibly both).  

THANKS

Credits go to a number of people: Steve Purkis for giving me namespace advice and James Keenan and Terrence Branno for their effort of keeping the CPAN tidier by making List::Utils obsolete.

Brian McCauley suggested the inclusion of apply() and provided the pure-Perl implementation for it.

Eric J. Roode asked me to add all functions from his module "List::MoreUtil" into this one. With minor modifications, the pure-Perl implementations of those are by him.

The bunch of people who almost immediately pointed out the many problems with the glitchy 0.07 release (Slaven Rezic, Ron Savage, CPAN testers).

A particularly nasty memory leak was spotted by Thomas A. Lowery.

Lars Thegler made me aware of problems with older Perl versions.

Anno Siegel de-orphaned each_arrayref().

David Filmer made me aware of a problem in each_arrayref that could ultimately lead to a segfault.

Ricardo Signes suggested the inclusion of part() and provided the Perl-implementation.

Robin Huston kindly fixed a bug in perl's MULTICALL API to make the XS-implementation of part() work.  

TODO

A pile of requests from other people is still pending further processing in my mailbox. This includes:
uniq_by(&@)

Use code-reference to extract a key based on which the uniqueness is determined. Suggested by Aaron Crane.

delete_index
random_item
random_item_delete_index
list_diff_hash
list_diff_inboth
list_diff_infirst
list_diff_insecond

These were all suggested by Dan Muey.

listify

Always return a flat list when either a simple scalar value was passed or an array-reference. Suggested by Mark Summersault.

 

SEE ALSO

List::Util  

AUTHOR

Tassilo von Parseval, <[email protected]>  

COPYRIGHT AND LICENSE

Copyright (C) 2004-2009 by Tassilo von Parseval

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.8.4 or, at your option, any later version of Perl 5 you may have available.


 

Index

NAME
SYNOPSIS
DESCRIPTION
EXPORTS
ENVIRONMENT
VERSION
BUGS
THANKS
TODO
SEE ALSO
AUTHOR
COPYRIGHT AND LICENSE

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Time: 04:14:16 GMT, September 24, 2010